1. Field of the Invention
The present invention relates to a vacuum ultraviolet (VUV) light source for generating an ultraviolet light by utilizing radiation light originating from a discharge plasma.
2. Description of the Related Art
In the case where an amorphous silicon thin film is formed through the direct dissociation of SiH.sub.4 by light, is necessary to use vacuum ultraviolet light.
A so-called .pi.-type discharge tube as shown in FIG. 10 is well known as a light source for generating vacuum ultraviolet light having a wavelength of below 180 nm.
This type of vacuum ultraviolet light source includes a cylindrical discharge tube 1 and a pair of electrodes 2 and 3 are located in the discharge tube in a spaced-apart relation. A window 4 is provided on one side end of the discharge tube 1 to take out ultraviolet light. A coolant passage 5 is provided outside the discharge tube 1 to allow a flow of a coolant. The discharge tube 1 has its inside adequately evacuated and a rare gas, hydrogen or denterium, is filled in the discharge tube 1 to an extent that a predetermined pressure can be maintained in the discharge tube. A power supply 6 supplies a power necessary for a discharge as generated between the electrodes 2 and 3. The power supply 6 charges a storage capacitor 8 through a DC power supply 7 and a switch 9 are used to apply pulse voltage to the electrodes 2 and 3.
When the pulse voltage is applied across the electrodes 2 and 3, a pulse discharge is generated across the electrodes 2 and 3 to radiate ultraviolet light through a plasma involved. The ultraviolet light is taken out from the window 4.
The conventional vacuum ultraviolet light source thus arranged poses the following problem. That is, in order to obtain high-output ultraviolet light, a high current density discharge is required for a current of quick rise time and high peak. In the aforementioned conventional light source a high current level results in an unstable discharge. It is, therefore, not possible to stably maintain the high-density discharge. Since, in the conventional light source, a discharge plasma cannot stably be created in a broader discharge space in a spatially uniform fashion, a corresponding ultraviolet light beam diameter is relatively small on the order of about 30 mm, largely restricting the application range of the light source.
In the conventional vacuum ultraviolet light source, therefore, the radiation efficiency and power output of the vacuum ultraviolet light are low and, therefore, it has been difficult to achieve a large-diameter ultraviolet light beam.